Integrated sensors have played an important role to the industry because they become the micro-sensing-organs for many automation processes. In the area of environment protection, reliable and cheap chemical vapor sensors are very attractive. Thus, recently the research and development of integrated acoustic sensors based on bulk and surface acoustic waves became of increasing interest. These acoustic sensors can determine concentrations of biological or chemical substances, electric and magnetic fields, temperature, and a wide range of mechanical properties such as acceleration, viscosity, etc. Most of these sensors operate in a gaseous medium, though a few are designed for use with liquids or solids.
The basic sensing mechanism of such acoustic devices is that if a parameter can alter the propagation characteristics of the acoustic waves such as velocity, amplitude or phase, then the change of the acoustic wave properties can then be correlated to the variation of this parameter. In other words, it is possible to monitor the change of this parameter by recording the acoustic property variation. In order to increase the sensitivity, differential sensing geometry is commonly adopted. This geometry requires two nearly identical acoustic wave paths, one being used as the sensing arm which is exposed to the intended measurand, and another as the reference arm. The differential output of these two arms is correlated to the variation of the measurand. The differential geometry is more reliable also as it is less sensitive to temperature and pressure variations or the like.
The operation principle of a common chemical acoustic sensor is that a coating is provided in the sensor arm of a differential acoustic sensor. This particular coating may absorb specific chemical vapor surrounding the device. The absorbed chemical vapor may increase the mass loading of the device, therefore change the properties of the propagating acoustic waves. These types of devices are called gravimetric sensors. A table below lists some examples of the coating and the measured chemical vapors or gases. A significant drawback of such sensors is that once the chemical vapor is trapped in the coating, it is very difficult to desorb it. That means that the device must be replaced after its use. It also means that the device must be disposable and cheap.
______________________________________ Material Number Coating to be sensed ______________________________________ 1 No Vapor (water, acetone, methanol) 2 Triethanolamine SO.sub.2 3 Palladium Hydrogen 4 WO.sub.3 H.sub.2 S 5 Polyethylene Cyclopentadiene maleate 6 Phthalocyanines NO.sub.2, NO.sub.3 7 Poly(dimethyl- Pentane, Hexane, siloxane) Iso-octane, Toluene ______________________________________
Previously, many bulk, surface and Lamb (plate) acoustic wave sensor devices are developed for such gravimetric sensing purposes. Lamb wave devices are proved to have highest sensitivity.